Recently, displays are being replaced with portable thin flat panel displays. Among the flat panel displays, an organic light-emitting diode (OLED) display which spontaneously emits light has a wide viewing angle, high contrast and fast response rate and is thus receiving attention as a next-generation display, and is also advantageous because of superior luminance, driving voltage and response rate and a wide color reproduction range.
Such flat panel displays have recently been manufactured to be light and thin and are thus portable and usable outdoors. In the case where a user sees an image outdoors using a flat panel display, sunlight is reflected from the display undesirably lowering contrast and visibility. Particularly, the case of an organic light-emitting display is problematic because such reflection from a metal reflective film is much more severe.
To solve this problem, a circular polarizer is typically disposed on one surface of an organic light-emitting display. Although the circular polarizer has a transmittance of about 40-45% and thus advantageously may reduce reflection of external light, light emitted from the inside of the device may be undesirably decreased. Furthermore, a typical circular polarizer consists of a linear polarizer and a 90° phase retarder, and has a total thickness of about 0.3 mm.
The case where this polarizer is applied to an ultrathin display which is the most recent trend is problematic because it may increase the thickness of the display and is also disposed outside the device thus requiring an additional process and making it difficult to form an integrated single device. For this reason, research into substitutes for the circular polarizer is currently ongoing.
According to recent studies (U.S. Pat. No. 5,049,780, U.S. Pat. No. 6,411,019), techniques for reducing reflection of external light using an optical interference filter in lieu of the circular polarizer have been devised. The optical interference filter using metal-dielectric thin layers enables light waves reflected from respective metal layers to disappear under conditions of phases therebetween being set to 180° by adjusting the thickness of dielectrics. In particular, when such thin layers are disposed on a bottom metal electrode layer of the organic light-emitting device, they play a role in reducing the reflection of external light from the metal electrode.
In addition, a method of coating a reflective electrode with a material able to absorb light has been exemplified (WO 00/25028, Optic Express V13. p. 1406 (2005), Thin Solid Films V379. p. 195 (2000)). In this method, examples of the material able to absorb light may include graphite, a black polymer, etc.
However, because the conventional configurations as above may decrease the reflectance of the metal electrode positioned under the light-emitting layer, light emitted downwards from the light-emitting layer is not reflected but is reduced, undesirably decreasing photoefficiency. Also, as the reflectance of the bottom metal layer positioned adjacent to the light-emitting layer decreases, a microcavity phenomenon may decrease undesirably lowering luminance. Thus, causes of lowering photoefficiency, which are the largest problems of the circular polarizer, are not removed.
To overcome such problems, recent research (U.S. Pat. No. 6,876,018) has proposed a configuration for reducing external light using destructive interference with the top metal layer without reducing the reflectance of the bottom metal electrode. In this case, however, to satisfy destructive interference conditions between the bottom metal electrode and the top metal layer, the thickness of an organic layer or a transparent electrode layer which is disposed between these two metal layers should be adjusted. When the thickness of the electrode layer and the light-emitting layer changes in this way, light emission efficiency and electrical properties may vary. Hence, the above technique is difficult to apply to actual devices because of influences on the electrical properties.